1. Field of the Invention
The present invention relates to a method of producing a silicon monocrystal in accordance with the Czochralski (CZ) method without performing a so-called necking operation.
2. Description of the Related Art
In the production of a silicon monocrystal in accordance with the Czochralski (CZ) method, a monocrystal of silicon has conventionally been used as a seed crystal. The seed crystal is brought into contact with silicon melt and is then pulled slowly while being rotated to thereby grow a silicon monocrystalline ingot. When the seed crystal is brought into contact with the silicon melt, slip dislocations are generated in the seed crystal at a high density as a result of thermal shock, and the dislocations propagate to the grown monocrystalline ingot. Therefore, in order to prevent the propagation of dislocations, there is performed a so-called necking operation in which the diameter of the crystal is decreased to about 3 mm, thereby forming a neck portion. Subsequently, the diameter of the crystal is increased to a desired diameter, and the silicon monocrystalline ingot is then pulled in a dislocation-free manner. The necking operation has been well known as a "Dash Necking Method," and use of this method has been a common practice in the case where a silicon monocrystal ingot is pulled in accordance with the CZ method.
Specifically, a conventional seed crystal is formed into a cylindrical shape having a diameter of about 8-20 mm or into a prismatic shape having sides of about 8-20 mm, and a cut-away portion or notch is formed for attachment to a seed crystal holder. The tip or lower end of the seed crystal, which end first comes into contact with silicon melt, is formed to have a flat surface. In order to safely pull a heavy monocrystalline ingot while sustaining the weight of the ingot, the seed crystal must have a dimension in the above-described range.
However, since the seed crystal having the above-described shape and dimension has a large heat capacity at the tip end which comes into contact with silicon melt, a steep temperature gradient is generated instantaneously within the crystal when the seed crystal comes into contact with the melt, so that slip dislocations are generated at a high density. Therefore, the above-described necking operation is required for growing a monocrystal while eliminating the dislocations.
However, in such a method, even when conditions for the necking operation are selected appropriately, the diameter of the crystal must be decreased to 4-6 mm or less in order to eliminate the dislocations. In such a case, the strength of the neck portion becomes insufficient to support a monocrystalline ingot whose weight has been increased with a recent increase in the diameter thereof, resulting in a high risk of fracture of the neck portion during the course of pulling of the monocrystalline ingot. This may result in a serious accident, such as dropping of the monocrystalline ingot.
In order to solve such a problem, the applicant of the present invention has proposed inventions disclosed in Japanese Patent Application Laid-Open (kokai) No. 5-139880 and Japanese Patent Application No. 8-87187. According to these inventions, the tip end of a seed crystal is formed into a wedge shape or is formed to have a hollow portion in order to reduce, to the extent possible, slip dislocation which would otherwise be generated when the seed crystal comes into contact with silicon melt. These inventions enable elimination of dislocations, even when the neck portion is formed to have a relatively large diameter, thereby increasing the strength of the neck portion.
Although the methods according to the cited inventions can increase the strength of the neck portion to some degree through an increase in the diameter of the neck portion, the methods still require performance of a necking operation, resulting in formation of a neck portion having slip dislocation. Therefore, in some cases, the strength of the neck portion of a monocrystalline ingot produced in accordance with either of these methods becomes insufficient for pulling the ingot if the monocrystalline ingot has a weight of 150 Kg or more as a result of recent increases in the diameter and length thereof. Accordingly, the methods do not thoroughly solve the problems involved in the prior art methods.
In view of the foregoing, the present applicant proposed a method of producing a silicon monocrystal, which method can make a growing crystal monocrystalline without performance of a necking operation for forming a neck portion, which would cause a problem in terms of strength (see Japanese Patent Application No. 9-17687). This method uses a seed crystal whose tip end has a sharp-pointed shape or a truncation thereof. The tip end of the seed crystal is gently brought into contact with the silicon melt, and the seed crystal is then lowered at a low speed in order to melt the tip end portion of the seed crystal until the size of the tip portion increases to a desired value. Subsequently, the seed crystal is pulled upwardly slowly in order to grow a silicon monocrystalline ingot having a desired diameter without performance of a necking operation.
According to this method, a contact area through which the tip end of the seed crystal is first brought into contact with the silicon melt is small, and the heat capacity of the tip end portion is small. Therefore, generation of thermal shock or a steep temperature gradient is prevented within the seed crystal, so that generation of slip dislocations is prevented. When the seed crystal is lowered at a low speed such that the tip end portion of the seed crystal is melted until the size of the tip portion increases to a desired value, a steep temperature gradient is not generated within the seed crystal. Therefore, no slip dislocation is generated within the seed crystal during the above-described melting operation. Finally, the seed crystal is slowly pulled upwardly in order to grow a silicon monocrystalline ingot. Since the seed crystal has a desired size and no dislocation, performance of a necking operation is not required, and the seed crystal therefore has a sufficient strength, allowing the seed crystal to be grown to a desired diameter to yield the silicon monocrystalline ingot.
As described above, in order to reduce the initial dislocation density, there have been proposed improved shapes for a seed crystal and improved necking methods which can maintain or increase the temperature of a seed crystal above the silicon melt or which can mitigate thermal shock upon contact with the silicon melt. However, since there is a limit to the thickness of a neck portion, the conventional techniques cannot cope with growth of a monocrystalline ingot having an increased diameter and an increased weight.
In view of this problem, there has been established the above-described dislocation-free seeding method; i.e., a method for bringing a seed crystal into contact with silicon melt for the purpose of initiating crystal growth, which does not require performance of a necking operation and which can cope with an increase in diameter and weight.
However, the problem involved in the dislocation-free seeding method is a rate of success in making a crystal dislocation free. That is, in this method, once a dislocation is introduced into a seed crystal, growth of a monocrystalline ingot cannot be continued or performed again unless the seed crystal is replaced with a new one. Therefore, increasing the success rate is considerably important. Further, even when seeding can be performed in a dislocation-free state, slip dislocation may be generated when the tapered tip end of a seed crystal is allowed to stand at a temperature near the melting point of silicon after the tip end has been melted to a predetermined distance, when the time required for initiating crystal growth or a growth rate during a transition period before the start of crystal growth is improper. The present inventors investigated the causes of such a phenomenon and found that even when the conventionally controlled factors such as shape of a seed crystal, a temperature maintaining time during which the seed crystal is held above melt (silicon melt), a melting speed, and a crystal growth rate are controlled appropriately, the above-described dislocation-free seeding cannot be performed with a sufficient success rate and with sufficient reproducibility.